Can someone let me know if I've got this right with the "My" possesion?
Yes, it's right!
Only for the Solar Class though, right? Also, only use "Ñuhys" if the noun in front of it starts with a "k", "p", "s", or "t".
If the plural adjective begins with those letters the z is changed by the s, and we would use the "Ñuhys" (which is normally singular, but in this case it would be used as plural). And yes, these are used only in the Solar Class.